1. Technical Field of the Invention
The present invention relates to a magnetic resonance-imaging system for medical use and a magnetic resonance signal acquisition method. More particularly, the present invention relates to a magnetic resonance imaging system which does not use an electrocardiographic (ECG) synchronizer per se, but substantially enables non-contrast MRA (MR Angiography) under ECG-gated trigger scanning and to a magnetic resonance signal acquisition method employed by the system.
2. Related Art
A magnetic resonance (MR) imaging is an imaging technique for magnetically exciting nuclear spin of a subject placed in a static magnetic field by using a high-frequency signal of Larmor frequency, and for reconstructing an image by using an MR signal generated with the excitation. Such magnetic resonance imaging systems implementing this imaging method have now become essential medical modalities.
In magnetic resonance imaging diagnosis for medical use, one of the important imaging techniques is MRA for depicting a blood vessel image of a subject. The MRA, for one thing, may be classified into contrast MRA and non-contrast MRA depending on whether or not a contrast medium is administered to a subject in carrying out the diagnosis.
The contrast MRA is an imaging technique in which MR scan is performed by administrating a contrast medium to a subject. This technique, however, casts a large mental and physical burden on a subject because an invasive treatment is required to be given to the subject in administrating the contrast medium. In addition, the cost for such an examination is expensive. On occasion, a contrast medium cannot even be administered to a subject depending on the constitution or the like of the subject. Therefore, use of non-contrast MRA is desired from the clinical viewpoint.
One of such non-contrast MRA techniques is performed by reflecting water components in blood. Japanese Unexamined Patent Application Publication No. H11-338409 discloses a technique included in this category, i.e., a SPEED (Swap Phase Encode Extended Data) technique, in which lung blood vessels having comparatively high flow speed are depicted utilizing the blurring of T2 relaxation time of blood. Japanese Unexamined Parent Application Publication No. H11-047155 discloses an FBI (Fresh Blood Imaging) technique, in which blood pumped out from the heart is scanned in a time phase of comparatively stable blood flow speed using an ECG synchronization technique.
These SPEED and FBI techniques have as their bases an FSE (Fast SE) technique. Thus, when the influence of the motion of a subject, which has been caused between echoes, on data acquisition is largely varied ghosts are likely to occur on a reconstructed image, thereby deteriorating image quality. It is important, therefore, that scanning is performed in a time phase when blood flow speed has been stabilized. In particular, when arteries are depicted, scanning should be performed in a time phase when a blood flow speed is comparatively low (i.e., the diastolic phase in a cardiac cycle). Accordingly, parallel use of the ECG synchronization technique is indispensable.
However, as taught by the foregoing known references, for the non-contrast MRA techniques by which the ECG synchronization technique is essential, a plurality of electrodes for detecting a signal of an ECG synchronizer are required to be stuck onto a subject, arising in problems, resolutions for which have been sought. Specifically, sticking a plurality of electrodes onto a body surface of a subject creates a heavy burden on an operator preparing for magnetic resonance imaging. For a subject, or a patient, as well, this sticking of electrodes may increase a mental and physical burden. Moreover, a gradient magnetic field signal for scanning may be superimposed on an ECG signal detected by the electrodes, which probably causes turbulence in the waveform of the detected ECG signal. If the turbulence in the ECG waveform becomes prominent, detection of the R wave may become difficult. This may deteriorate the quality of a reconstructed image, and require time, more than necessary, for scanning in the magnetic resonance imaging technique, necessitating one to start over preparation for setting, scanning and the like again and again. As a result, patient throughput is decreased.